Synthesis of Zwitterionic Triphosphenium Transition Metal Complexes: A Boron Atom Makes The Difference

A collection of zwitterionic phosphanide metal carbonyl coordination complexes has been synthesized and fully characterized, representing the first isolated series of metal complexes for the triphosphenium family of compounds. The dicoordinate phosphorus atom of the zwitterion is formally in the +1 oxidation state and can coordinate to one metal, 2M (M = Cr, Mo, W) and 2Fe, or two metals, a Co2(CO)6 fragment 4, depending on the starting reagents. All complexes have been isolated in greater than 80% yield, and structures were confirmed crystallographically. Metrical parameters are consistent with 1 being a weak donor and results in long metal–phosphorus bonds being observed in all cases. Unique bimetallic structures, 3M (M = Cr, Mo, W), consisting of a M­(CO)5 fragment on phosphorus and a piano-stool M­(CO)3 fragment on a boron phenyl group have been identified in the 31P­{1H} NMR spectra and confirmed using X-ray diffraction studies. Use of the borate backbone in 1, which renders the molecule zwitterionic, proves to be a determining factor in whether these metal complexes will form; the halide salt of a cationic triphosphenium ion, 6[Br], shows no evidence for formation of the analogous metal complexes by 31P­{1H} NMR spectroscopy, and tetraphenylborate salts, 6[BPh4] and 7[BPh4], produce complexes that are unstable.